This application is based on application No. 2000-270447 filed in Japan, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical linkage device for use in optical transmission apparatus such as a medium converter, the optical linkage device operative to convert an electrical signal into an optical signal.
2. Description of the Related Art
The optical linkage device employs a laser diode which is significantly varied in output characteristics thereof and hence, the fabricated optical linkage device requires adjustment for the output of the laser diode before shipping.
According to the conventional practice, adjustment of a trimmable resistance on a board is made in a state where a case of the optical linkage device is opened or where the board is dismounted from the case, and then the case is mounted to place to complete the optical linkage device.
After the adjustment, however, the fixing of the case may cause the variation of internal temperature conditions of the linkage device. Consequently, the conventional technique cannot obviate the variations of laser output characteristics associated with the increased internal temperature of the fabricated optical linkage device.
It is an object of the invention to provide an optical linkage device which permits the adjustment of output of a light emitting element to be performed with the case fixed to place thereby ensuring a more stable product quality.
In accordance with the invention, an optical linkage device comprises: a light emitting element; a trimmable resistance for controlling output characteristics of the light emitting element; a board having the trimmable resistance mounted thereon; and a case at least covering the board, wherein the case is provided with an adjustment hole for adjustment of a resistance value of the trimmable resistance.
According to the above arrangement, the resistance value of the trimmable resistance can be controlled by adjusting an adjustable member via the adjustment hole in a state where the case is mounted on the board.
In a case where a substantial distance cannot be attained between the case and the board, a distal end of the adjustable member of the trimmable resistance may be positioned at some intermediate point relative to a thickness of the case. In this case, a distance between an inside surface of the case and the board is relatively decreased so that the whole body of the optical linkage device can be reduced in size.
In another feature of the invention, the adjustment hole may be formed in a top surface of the case. Where the optical linkage device is mounted to a housing of a higher rank unit such as a medium converter, the adjustment can be performed via the top surface of the case if space above the top surface of the case is available.
In another feature of the invention, the adjustment hole may be formed in a lateral side of the case. In a case where there is no space available above the top surface of the case and an available space is provided only laterally of the case, the adjustment may be performed via the lateral side of the case.
In another feature of the invention, a ventilating through-hole may be provided at any one of the board, the top surface of the case and the lateral side of the case. In this case, the adjustment hole and ventilating through-hole may serve as heat releasing holes capitalizing on convective air flow. Thus, heat release efficiency is increased so that performance deterioration associated with elevated temperatures is prevented.
As described above, the invention permits the resistance value of the trimmable resistance to be controlled via the adjustment hole with the case assembled in place. Therefore, the output characteristics of the optical linkage device can be adjusted in a like state to an actual operation environment.